Abstract The adipocyte is a potent endocrine cell, communicating with most systemic cells directly or indirectly. Over the course of the previous funding period, we have focused on adipokines and had a lot of emphasis on inflammatory factors. While we plan to maintain this focus in the continuation of our work, we will focus much more on lipid mediators and metabolites produced by adipocytes, used in the communication axis between adipose tissue, the liver and the brain. We uncovered an entirely new level of metabolic regulation in the adipocyte that is the result of an altered rate of uridine biosynthesis based on our work in the ongoing funding period on Xbp1s, a mediator of the unfolded protein response (UPR). We discovered that endogenous plasma uridine concentrations are regulated by fasting-refeeding in rodents. Importantly, this response is conserved in humans. Enhanced uridine biosynthesis in the adipocyte is an adaptive response to fasting, a process dysregulated under conditions of obesity and diabetes. Xbp1s, an ER stress transducer, serves a master regulator of the fasting response in adipocytes, transcriptionally activating multiple enzymes, including CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamylase, dihydroorotase), the rate-limiting enzyme for uridine biosynthesis. We have found that Xbp1s is induced in the fed state in the liver, and Xbp1s is induced in the fasted state in adipocytes. These findings helped us define a previously unrecognized axis between the adipocyte, the liver and central neuronal targets. Activation of the uridine biosynthetic pathway in adipocytes leads to enhanced lipolysis and increased ? oxidation. We will take advantage of the ability to manipulate upstream mediators of the Xbp1s pathway, particularly free fatty acids, and will address a) how these phenomena are affected by central lipid sensing mechanisms and how uridine signals affect critical brain regions and b) how the resulting adipocyte signals affect hepatic homeostasis. The three Specific Aims are designed to directly asses the role of local Xbp1s responses in adipose tissue, which translate into systemic consequences via actions on the liver and/or key neurons. Specific Aim 1: To determine whether adipocyte- specific manipulation of CAD, the rate-limiting enzyme for de novo uridine synthesis, has a physiological impact on adipose tissue physiology and systemic metabolic flexibility; Specific Aim 2: Assess the physiological importance of lipid sensing and transport via TLR4 and CD36 for adipose tissue Xbp1s activation and its impact on uridine biosynthesis. Specific Aim 3: Evaluate the physiological effects of uridine on central and peripheral leptin signaling. Notably, we are focusing on an entirely new system for the adaptive metabolic response at the interface of feeding and fasting. The adipocyte moves center stage in this process, prompting hepatic and neuronal responses to adapt to altered nutrient conditions. We propose that uridine serves as adipocyte-derived signal and hope to unravel the molecular events in the liver and in the brain with the help of the other expert investigators assembled here.